This invention relates to an apparatus for improving characteristics of sand, especially sea sand, utilized to prepare green concrete and concrete secondary products for the purpose of preventing degradation with time of the characteristics of concrete structure caused by salt hazard or the like and for improving the mechanical strength of concrete structures, and more particularly, an apparatus for obtaining sand containing a small quantity of impurities with a constant surface water content by removing mud and salt components contained in material sand by centrifugal separation together with surface water while washing with water.
In recent years, degradation of the quality of green concrete or concrete secondary products utilizing sea sand, which is caused by salt hazard, presents serious problems so that it became essential to remove salt when using sea sand.
Heretofore salt has been removed by sprinkling clean water on sand collected from sea bottom while the sand is carried by a transport ship (on-board water sprinkling method) or by sprinkling clean water onto sand piled up on land (land water sprinkling method).
With these prior art methods, however, since water is sprinkled onto a large quantity of sand, water is not sprinkled uniformly thus failing perfect salt removal. Conversely, for perfectly removing salt, it is indispensable to consume a larger quantity of clean water than actually necessary. Furthermore, in the case of the on-land sprinkling method, a large land area is necessary. Further, immediately after water sprinkling, the percentage of the surface water of sand is high so that such sand can not be used to prepare green concrete, thus requiring certain time for drying under sun light.
In a green concrete factory and a secondary product manufacturing factory, sand is stored in the outdoor where rain falls, whereby the surface water content of sand varies always. Accordingly, before using sand, it is necessary to measure the surface water content and correct the quantity of admixed water, which is troublesome and requires additional labor.
For this reason, after sprinkling a definite quantity of water onto sand, the sand is projected against a plate by using a rotary drum, a rotary table or the like (collision plate method).
With this method, however, separation of the surface water is effective only in a moment at which the sand collides upon the plate. This method is effective for coarse sand but not effective for fine sand, because the surface area of the sand is relatively large and the surface tension of the surface water is large. Though, the surface water is separated from the sand at an instant when the sand collides upon the collision plate, the separated water adheres again to sand, since the separated water scatters as a result of collision, thereby making it difficult to efficiently remove water from sand. This tendency becomes remarkable as the percentage of surface water increases. Furthermore, the quantity of the surface water after separation varies depending upon the quantity of the initial surface water so that the sand is crushed by the shock at the time of collision, thus changing the physical characteristics of the sand.
With regard to the desalting effect, the quantity of salt decreases only in proportion to the quantity of separated surface water, so that the desired object can not be attained.